1.) Defeintly get at LEAST one more box, a big one so they have PLUNTY of room, or a covered big one, as big as you can find.
Otherwise, specially as they get older your cats can develope some rather unwanted improper elimination habits.

2.) Yes you can use baking soda to help elminate odors.

3.) Scoop at least 2 times a day.

4.) You can spray a little bit of Pam (cooking spray) into CLEAN boxes before adding the litter in, so that litter wont stick so bad to the pan itself.

5.) Empty and scrub down your boxes with a bleach/water combo and rinse well 1-2 times every month (or more if needed.)

6.) Keep about 2-3 inches of litter in each box at all times.

7.) Bad smells can be caused by bad food, what food are you feeding them, often the low grade brands can cause horrid smells! I can't have things like Frisky's, Meow Mix, or Purina in my house because of that! Often a high grade diet will help with stinkies.

8.) Try a change of litter!!
I recommend..
- Papurr
- Scoop Away + Crystals
- Arm n' Hammer + Crystals.